create table b$m_test (ClientID int, CourseID int);

insert into b$m_test values(1,1);
insert into b$m_test values(1,2);
insert into b$m_test values(1,3);
insert into b$m_test values(2,1);
insert into b$m_test values(3,2);
insert into b$m_test values(4,1);
insert into b$m_test values(4,2);

select count(1) as cnt
  from (
         select ClientID
           from b$m_test
          group by ClientID
         having count(distinct CourseID) > 1
       ) v